The prior art has used devices such as a bent sub and bent housing whereby it is possible to assemble two sections of drill string so that the axis of one section will be at an angle to the axis of another and adjacent pipe section. Such devices have been employed in drilling deviated bore holes whereby the vertical inclination of the hole may be changed.
The bent sub consists of a tool joint. The tubular section has threaded ends. The axis of one of the threaded ends is coincident with the axis of the tubular section. The axis of the opposite end is at an angle to the axis of the tubular section.
When the bent sub is used to connect pipe sections, such as drill pipe stands, the axis of adjacent stands will have the angle imposed with respect to each other established by the bent sub.
The bent sub, when a down-hole motor is used, usually is placed between the drill collars which are used to impart the desired weight on the bit and the down-hole motor.
The permissible angle which may be established depends on the diameter of the bore and the diameter of the bit. The angle may not exceed that which will cause the bit to cut into the casing or bore hole by dragging along the casing or the bore hole through which it passes.
The initial angle may not, however, be the maximum angle permitted by the bore hole. The angle is also determined by the rate at which the vertical angle of the bore hole axis is to change with depth to give the arc which the drilling program demands for the bore hole to reach its objective.
The rate of build-up of angle is, however, also determined by the pendulum effect of the drill pipe which has a tendency to reduce the vertical angle of the bore hole. It thus may require an increase or decrease in the angle of the bent sub as the bore hole progresses.
These considerations require an inventory of bent subs of different angles as the hole progresses; and when the bent subs are to be interchanged to increase or decrease the angle, the drill string must be disassembled at the bent sub and a different bent sub introduced.